Real time clock generators have various applications. They can be used for providing timing information for operating systems, for enforcing policies for time-sensitive data, for assisting in positioning calculations and the like. U.S patent application serial number 2002/0083284 of Matsubara et al., titled “Data reproduction system, data recorder and data reader preventing fraudulent usage by monitoring reproducible time limit”, U.S. Pat. No. 5,920,727 of Kikinis et al., titled “Timer-controlled computer system shutdown and startup”, U.S patent application serial number 2004/0128528 of Poisner titled “Trusted real time clock” and U.S patent application serial number 2004/0225439 of Gronemeyer, titled “Method and apparatus for real time clock (RTC) brownout detection, all being incorporated herein by reference, illustrate some usages of real time clocks.
Real time clock signals can be tampered for various reasons including copyrighting piracy, concealing hacking or tampering attempts, reducing the functionality of a device and the like.
One tampering method involves repetitive alterations of the supply voltage level provided to the real time clock generator, in order to force the real time clock generator to reset or to get stuck.
The supply voltage can be provided by a so-called “external” voltage supply unit—a voltage supply unit that is located outside an integrated circuit that includes the real time clock.
Monitoring of these external voltage supply units can be power consuming and during low power modes the monitoring should be stopped.
There is a need to provide efficient methods and devices for protecting a real time clock.